This invention relates to an optical transmitting apparatus, and more specifically, to an optical transmitting apparatus for modulating a phase and a polarization of a signal light in advance in an optical transmitter of a long distance optical transmission system.
In general, in a long distance optical transmission system, especially in a long distance wavelength division multiplexing optical transmission system, it has been necessary to modulate a phase of a signal light pulse for improving their transmission characteristics and to scramble polarization of the pulse for suppressing polarization dependence of the transmission system before transmitting the pulse to the transmission line. See, for instance, F. Heismann et al. xe2x80x9cElectrooptic Polarization Scramblers for Optically Amplified Long-Haul Transmission Systemsxe2x80x9d, IEEE Photonics Technology Letters, vol. 6, No. 9, September 1994 and N. S. Bergano et al. xe2x80x9cBit-synchronous polarization and phase modulation scheme for improving the performance of optical amplifier transmission systemsxe2x80x9d, Electronics Letters 4th January 1996, vol. 32, No. 1.
Therefore, in a conventional optical transmitter, an optical intensity modulator or a data modulator intensity-modulates a CW laser light into NRZ pulses or RZ pulses in accordance with a data to be transmitted, and then a polarization scrambler simultaneously modulates the phase and polarization of the pulse. However, as the phase modulation volume achieved by the polarization scrambler is equal to the average amount of the phase modulation of the principal axis and the secondary axis, it requires voltage higher (approximately 1.4 times higher) than that used in an ordinary phase modulator. In order to provide the optimum phase modulation, an extra phase modulator should be disposed separately from a polarization modulator.
Although a lithium niobate crystal is generally employed for the phase modulator and the polarization modulator or polarization scrambler, its usage is different from each other. When the lithium niobate crystal is operated as the phase modulator, a polarization of incident light is adjusted to the direction of the maximum modulation efficiency. On the other hand, when it is operated as the polarization modulator or polarization scrambler, the polarization direction of incident light is set at an angle of 45xc2x0 to that of the highest modulation efficiency. Although the function of the polarization scrambler is basically to rotate the polarization in an optical pulse, it secondarily modulates the phase.
The phase modulation and the polarization modulation in the optical transmitting apparatus are referred in Japanese Patent Open Disclosure Gazettes No. 8-111662, 8-237224, 8-237225, 9-197354, 9-200128, 9-233029 and 10-75216. Especially, inventions described in the Japanese Patent Open Disclosure Gazettes No. 8-111662, 8-237224, 8-237225 and 9-197354 propose to make the residual degree of polarization substantially zero and the objects of the inventions in the other Gazettes are fundamentally the same. In the meanwhile, the degree of polarization is generally defined as the ratio of optical power of a polarization component to all optical power. This specification also adopts this definition.
When the extra phase modulator is disposed separately, the apparatus has to comprise the three modulators of the data modulator, phase modulator and polarization modulator. Thus, the apparatus becomes large-sized as well as high-priced causing a rise of trouble in production and maintenance.
As described above, in a conventional optical transmitting apparatus combining the phase modulator and the polarization modulator, the polarization is scrambled so that the degree of residual polarization or the average degree of polarization becomes zero. However, it has been proved from the subsequent research that to make the residual polarization zero does not necessarily improve the transmission characteristics in an RZ transmission.
An object of the invention is to provide an optical transmitting apparatus for effectively modulating a phase and a polarization of a signal light.
Another object of the invention is to provide an optical transmitting apparatus to be produced at lower price and smaller size compared with a conventional apparatus.
A further object of the invention is to provide an optical transmitting apparatus capable of obtaining the better transmission characteristics in an RZ transmission.
In this invention, a phase modulator modulates a phase of a signal light pulsated according to the data to be transmitted and outputs it to a birefringent material at an angle of 45xc2x0 to the principal axis.
Since the birefringent material changes the polarization of the input signal according to its phase, the obtained result becomes substantially the same with that obtained by the polarization scrambling. As the phase modulator is used at the maximum modulation efficiency, the phase modulation can be carried out more efficiently. It is highly reliable because no driving source is required for the polarization modulation and thus the polarization modulation also can be done efficiently. Therefore, it is possible to make the phase modulation and the polarization modulation more efficient and also independently, and, as a result, a highly reliable optical transmitting apparatus can be realized.
The birefringence material comprises for example a polarization-preserving fiber and it is applicable either way to connect directly with an output of the phase modulator or to connect with the birefringence material through a polarization rotator.
Also, this invention uses a travelling wave type phase modulator wherein the signal light phase-modulated at the phase modulator is rotated so that its polarization direction becomes at an angle of 45xc2x0 to the principal axis and sending it back to the phase modulator again. By this structure, the phase modulator can be used for both phase modulation and polarization modulation. Consequently, a low-priced optical transmitting apparatus can be realized and also its configuration can be simplified.
In order to pick up the phase-modulated and polarization-modulated signal light, for example an optical circulator should be employed. The optical circulator can introduce the signal light to be modulated into the phase modulator and efficiently extract the phase-modulated and polarization-modulated signal light output from the phase modulator toward the outside. The phase modulator comprises for example a LiNbO3 crystal.
As the phase-modulated signal lights of different wavelengths are polarization-combined and sen t to the birefringence material, the multiple wavelengths can be polarization-modulated spontaneously. The number of birefringent mediums for obtaining the wavelength division multiplexed signal light can be reduced.
If the polarization directions of the signal lights of adjacent wavelengths are set so as to meet orthogonally, the number of the required birefringent mediums can be reduced and also the number of dispersion compensators can be reduced by half when the dispersion compensation is needed. Those reductions contribute to miniaturize the whole structure.
The optical transmitting apparatus according to the invention also comprises a signal light generator for generating an RZ pulse light in accordance with a data to be transmitted; a phase modulator for phase-modulating the RZ pulse signal light output from the signal light generator by synchronizing with the RZ pulse signal light; and a polarization modulator for polarization-modulating the output light from the phase modulator, wherein the degree of residual polarization in the output from the polarization modulator is set to a non-zero finite value. Consequently, satisfactory transmission characteristics can be obtained in the RZ optical transmission system without such excess phase modulation as setting the degree of the residual polarization to zero. The degree of the residual polarization in the output from the polarization modulator is preferably from 20% to 70%.
The optical transmitting apparatus according to the invention also comprises a data output means for outputting a signal indicating a data to be transmitted; an RZ light source for generating an RZ pulse signal light according to the output data from the data output means; a phase adjuster for adjusting a phase of a clock substantially synchronized with the output data from the data output means; a phase modulator for modulating a phase of the output light from the RZ light source; and a polarization modulator for modulating a polarization of the output light from the phase modulator according to the phase of the output light from the phase modulator.
Because the phase adjuster is disposed, the degree of polarization modulation, namely the degree of the residual modulation can be determined independently from the degree of phase modulation.
The polarization modulator comprises for example a birefringent material, or comprises a polarization controller for controlling a polarization of the output light from the phase modulator and a birefringent material into which the output light from the polarization controller inputs at a predetermined angle to the principal axis.
If the degree of residual polarization in the output light from the polarization modulator is set to non-zero, for example from 20% to 70%, the satisfactory transmission characteristics can be obtained without the excess phase modulation in the RZ optical transmission.
The phase modulation timing of the phase modulator should be set so that the direction of frequency deviation inverts around the peak of the input RZ pulse. Accordingly, it becomes capable of giving the appropriate polarization modulation and also capable of optimizing the degree of phase modulation.